1. Field of the Invention
The present invention relates to an optical transmission substrate employed for optical interconnection or the like, and a method for fabricating the same, and particularly, an optical transmission substrate which includes an optical transmission body provided in a through hole having excellent and uniform transmission characteristics. The present invention also relates to an optoelectronic hybrid substrate provided with the optical transmission substrate.
2. Description of the Background Art
The operation speed and number of electric-signal I/O terminals of a semiconductor device will continue to rise for the purpose of raising throughput and processing speed in information processing. At the same time, the number of signal lines in a circuit substrate for mounting such a semiconductor device tends to significantly increase and the electric line density will also rise. This causes the serious problem of sharply increasing signal damping in electric line formed in a mounting substrate and crosstalk between adjacent lines. Especially in a large-scale semiconductor integrated circuit mainly including a microprocessor, a GHz-level signal needs inputting and outputting stably with a low power consumption.
In order to solve the problems, the optical transmission art has been studied of converting an electric signal inputted in and outputted to a semiconductor device into an optical signal and transmitting light corresponding to the optical signal through optical line such as an optical waveguide formed in a mounting substrate.
In a photoelectric conversion section converting an electric signal into an optical signal, a light-emitting optical semiconductor device such as a semiconductor laser (LD) and a light-emitting diode (LED) mainly made of a compound semiconductor is used on a transmission-output side while a light-receiving optical semiconductor device such as a photo-diode (PD) made of a silicon (Si) or a compound semiconductor is used on a reception-input side.
Among various semiconductor lasers, a surface emitting laser (VCSEL) allowing a light-emitting section to emit light perpendicularly to the main surface of a device substrate has been recently widely employed as a high-performance and low-cost transmission light source. This is because the surface emitting laser is capable of obtaining an excellent crystal on a crystal growth plane thereof. As a photo-diode alike, a surface-receiving type having a light-receiving section arranged on a crystal plane thereof has been generally employed.
Conventionally, an optical transmission substrate is known which includes an optical waveguide formed by covering a core made of a high refractive-index material with a clad made of a low refractive-index material as optical line parallel to the substrate surface on or inside of the substrate. The optical waveguide is formed by an optical glass or a single-crystal or polymeric optical material.
In addition, a conventional optoelectronic hybrid substrate hybridizing optical line and electric line is known which has an optical-coupling structure of the above optical semiconductor device and optical waveguide. In the optoelectronic hybrid substrate, the I/O direction of signal light is almost perpendicular to the optical waveguide formed on a mounting substrate, thereby presenting various proposals for obtaining a greater coupling light quantity.
FIG. 10 is a sectional view showing a typical example of a conventional optical transmission substrate having an optical waveguide and an example of an optical-electric circuit substrate disclosed in Japanese Patent Laid-Open Publication No. 2003-50329. In the example of FIG. 10, an optical line layer (optical waveguide) 103 and electric line 105 are formed on a substrate 100. As shown by a broken line in the figure, signal light emitted from a laser diode 101 on the transmission side: is incident vertically upon an upper clad 103b forming a part of the optical line layer 103 and enters into a lower clad 103c through a core pattern (core) 103a, and then the signal light turns the propagation direction to the line direction along the optical line layer 103 at a mirror member 104 arranged in the optical line layer 103; and is incident upon the core pattern 103a of the optical line layer 103.
Likewise on the reception side, the signal light propagating through the core pattern 103a of the optical line layer 103 reaches the lower clad 103c once, turns upward vertically to the optical line layer 103 at the mirror member 104, and similarly, is incident upon a photo-diode 102 through the core pattern 103a and the upper clad 103b. 
Furthermore, Japanese Patent Laid-Open Publication No. 2004-54003 discloses an example of a conventional optical transmission substrate in which a short optical fiber as an optical waveguide is embedded in a through hole formed in a plurality of laminated dielectric layers.
Moreover, Japanese Patent Laid-Open Publication No. 2004-279687 discloses an example of a conventional optical transmission substrate and optoelectronic hybrid substrate including a plurality of laminated dielectric layers provided with an optical waveguide formed in a through hole.
However, the configuration of Japanese Patent Laid-Open Publication No. 2003-50329 shown in FIG. 10 has the problem of making harder in efficiently coupling the surface-type optical semiconductor device of the laser diode 101 and the photo-diode 102 optically with the optical waveguide of the optical line layer 103. Specifically, signal light emitted from the laser diode 101 widens by a half-value total angle of tens degrees into a spot size several or more times as large as that at the emission point when reaching the mirror member 104 through the optical line layer 103.
Even after undergoing an optical-path conversion at the mirror member 104, the signal light radiates while propagating through the lower clad 103c covering the reflection plane, thereby making the spot size of the optical signal several to tens times as large as that at the emission point when reaching the core pattern 103a of the optical line layer 103, far larger than the core pattern 103a having a cross-section size of tens-μm angle.
This hinders the signal light from being efficiently incident upon the core pattern 103a and thus lowers the signal-light transmission level in the optical line layer 103, thereby causing the problem of making harder in raising the signal-to-noise ratio (S/N ratio) or the dynamic range of signal modulation.
In order to solve the problem, the signal-light transmission level needs to be raised, and thus, the electric current injected into the laser diode 101 needs increasing to heighten the light output. However, this leads to an increase in the power consumption of the laser diode 101, thereby lowering the energy efficiency of signal transmission.
At the same time, the increased electric current injected into the laser diode 101 generates more heat in the laser diode 101, thereby requiring an additional complex radiation structure and deteriorating the reliability. Besides, heat radiated from the substrate 100 may adversely affect the operation of a system provided with the optical-electric circuit substrate.
In addition, the optical transmission substrate disclosed in Japanese Patent Laid-Open Publication No. 2004-54003 includes the short optical fiber embedded in the through hole, thereby excessively increasing the process-hour for the optical transmission substrate provided practically with a great deal of optical line and thus worsening the mass productivity. Instead of embedding the short optical fiber in the through hole, another method is known of directly forming an optical transmission body shaped like an optical waveguide in a through hole. This method also has the problem of making harder in forming an optical transmission body having a uniform structure in the optical transmission direction in a thicker optical transmission substrate, thereby causing a greater optical-transmission loss.
Furthermore, in the optical transmission substrate disclosed in Japanese Patent Laid-Open Publication No. 2004-279687, in forming the optical waveguide in each through hole of the plurality of laminated dielectric layers, a continuous core is formed over the whole laminated layers by applying a writing beam from between each dielectric layer after laminating the dielectric layers. In this case, a generally-narrow optical-coupling-efficiency tolerance to a shift in the connection part of each optical waveguide formed in each through hole of the layers may cause a greater optical-transmission loss from a shift in adjacent optical waveguides through a subsequent fabrication process or the like. Besides, the optical transmission substrate cannot be applied to a design created by deliberately shifting the optical waveguides of adjacent layers.